In a typical proton therapy system used for tumor radiation treatment for example, a proton beam is generated and output from an accelerator, e.g., a cyclotron or a synchrotron, with a certain initial energy. The initial energy determines a maximum penetration depth of the proton beam and typically is 250 MeV. As the proton beam travels through a beam transportation system or a beamline, the beam energy is precisely tuned through energy selection mechanisms, e.g., an energy degrader or energy slit. The beam transport system includes a plurality of magnets for beam redirection (bending), focusing and steering. A rotational gantry with a radiation nozzle (or beam nozzle) is located at the end of the beam transport system. Eventually, the beam is delivered to a treatment station and irradiated onto a patient at an energy level prescribed for the specific treatment session based on the tumor volume, geometry, location etc.
Due to the extremely high cost of purchasing and maintaining such a radiation system, a medical facility usually uses one accelerator for a plurality of treatment stations so the high expenditure for the accelerator facilities is distributed. Although using a multi-station single-cyclotron system is effective to distribute the cost for large medical facilities, the overall cost of such a multi-gantry system can be prohibitively high for smaller facilities that may only need one treatment station. Also, some multi-station systems do not support simultaneous treatment in multiple stations. This contribute to further disadvantage that a delay at one treatment station can cause delay at the other station.
With the demand for proton beam radiation therapy rising worldwide, smaller and less expensive proton therapy systems are highly desired to increase patient access to therapy. In a proton radiation system, a gantry system alone typically weighs over 200 tons which is mainly contributed by the massive magnets installed in the gantry. To support and precisely control the motion of such a large weight, existing rotatable gantries are supported by a front and a rear ring structures, between which the components in the gantry beamline are suspended. That is, the gantry is supported in a simply-supported manner, in that the weight of the gantry is supported at both end portions of the gantry.
Typically, the front and rear rings are respectively coupled to front and rear rollers that can rotate with the gantry. The rings and rollers are affixed to the ground through fixtures which are made of steel and concrete. The two rings, plus the additional structural members to stiffen the assembly, make the gantry system appear to be a tremendously large and heavy conical (generally tapering from right to left as illustrated) or cylindrical drum-shaped structure in a treatment station. This structure defines the overall size of the gantry, such as the end-to-end gantry length. It also undesirably limits patient positioning and makes it difficult to make incremental improvements to the beam optics geometry because the critical dimensions are determined by the mechanical structure, not by the magnet positioning.
More importantly, using two rollers to support introduces random deflection errors during rotation due to the inherent difficulty in aligning the two rollers perfectly. As a result, the gantry tends to precess, causing the beam spot location to shift in an unpredictable manner. In practice, remarkable time and resources are often spent on realigning and repositioning the rollers in the attempt to fix the random errors and maintain beam precision.
Moreover, a typically rotatable gantry is capable of rotating in a range greater than 180° such that the beam nozzle can irradiate toward a patient in various prescribed angles, e.g., both from above the patient table and from underneath the patient table. It is desirable that the floor in the treatment station can safely support a clinician's access to the gantry and the patient table for treatment preparation without interfering with gantry movement during treatment.